In CVJ, stresses are repeatedly applied to the balls thereof and the surface of metals which are brought into close contact with the balls, due to complicated reciprocating motions with rolling and sliding observed under a high contact pressure during the rolling motions thereof and accordingly, a problem arises such that the CVJ is quite liable to cause a flaking phenomenon due to fatigue of metals.
Examples of conventionally used grease compositions for use in CVJs include lithium soap thickened extreme-pressure grease compositions comprising molybdenum disulfide; lithium soap thickened extreme-pressure grease compositions comprising molybdenum disulfide and sulfur-phosphorus extreme-pressure agent or lead naphthenate (see, for instance, Non-patent Document 1 specified below).
Furthermore, Patent Document 1 also discloses a grease composition which comprises at least one member selected from the group consisting of divalent typical metal oxides having a Mohs hardness lower than that of the steel and compounds of the foregoing divalent typical metals, which can immediately be converted into the corresponding oxides of these metals having a Mohs hardness lower than that of the steel under the boundary lubrication conditions.
Patent Document 2 discloses a grease composition for use in constant velocity joints, which comprises a urea thickener, molybdenum disulfide, a calcium salt such as that of oxidized wax or that of sulfonic acid, and molybdenum dithiocarbamate.
Patent Document 3 discloses a grease composition for use in constant velocity joints, which comprises a urea thickener, an organic molybdenum compound such as molybdenumdithiocarbamate or molybdenum dithiophosphate, and a calcium salt such as that of oxidized wax or that of sulfonic acid.
However, these grease compositions for CVJs are not necessarily satisfied when they are used under such severe operating conditions generated in the recent high-performance motorcars because of the following reasons.
The cross-groove type joint and the double offset type joint used as the plunging type constant velocity joint as well as the bar field type joint used as the fixed type constant velocity joint have a structure in which six balls in general transmit torque.
On the other hand, the load applied onto the balls or the area which comes in contact with the same has been increased due to the miniaturization of the size of the CVJ along with the recent weight-reduction of the motorcar. Such an increase in the load is one of the reasons why the joint would easily undergo flaking phenomena. If the balls or the contact area undergo such flaking, it would be difficult to smoothly transmit torques through the joint and therefore, this becomes a cause of vibrations, noises and abnormal sounds of motorcars. As has been discussed above, the size of the CVJ has been reduced along with the miniaturization of the motorcars and accordingly, the resulting contact pressure would relatively be increased. For this reason, the conventional grease compositions cannot satisfactorily control or eliminate the occurrence of the foregoing flaking phenomenon.                Patent Document 1: JP-A-2003-183687;        Patent Document 2: JP-A-Hei 9-194871;        Patent Document 3: JP-A-Hei 9-324190;        Non-Patent Document 1: Fish, G., Constant Velocity Joint Greases, Euro Grease, 1997, May/June, 25.        